1. Field of the Invention
The present invention relates to polarizing optical elements for use in the visible portion of the electromagnetic spectrum. More particularly, the present invention relates to imbedded or immersed wire grid polarizers that efficiently transmit light of a specific polarization while efficiently reflecting light of the orthogonal polarization.
2. Prior Art
Because wire grid polarizers are wavelength sensitive optical devices, imbedding the polarizer in a material or medium with an index of refraction greater than one will always change the performance of the polarizer over that available in air for the same structure. Typically, this change renders the polarizer unsuitable for the intended application. Imbedding the polarizer, however, provides other optical advantages. For example, imbedding the polarizer may provide other beneficial optical properties, and may protect the polarizer, although the performance of the polarizer itself, or polarization, may be detrimentally effected. Therefore, it is desirable to obtain the optimum performance of such an imbedded wire-grid polarizer.
Wire grids are typically disposed on an outer surface of a substrate, such as glass. Some wire grids have been totally encased in the substrate material, or glass. For example, U.S. Pat. No. 2,224,214, issued Dec. 10, 1940, to Brown, discloses forming a polarizer by melting a powdered glass packed around wires, and then stretching the glass and wires. Similarly, U.S. Pat. No. 4,289,381, issued Sep. 15, 1981, to Garvin et al., discloses forming a polarizer by depositing a layer of metallization on a substrate to form the grid, and then depositing substrate material over the grid. In either case, the wires or grid are surrounded by the same material as the substrate. As stated above, such encasement of the wires or grids detrimentally effects the optical performance of the grid.
U.S. Pat. No. 5,748,368, issued May 5, 1998, to Tamada et al., discloses a narrow bandwidth polarizer with a grid disposed on a substrate, and a wedge glass plate disposed over the grid. A matching oil is also applied over the elements which is matched to have the same refractive index as the substrate. Thus, the grid is essentially encased in the substrate or glass because the matching oil has the same refractive index. Again, such encasement of the grid detrimentally effects the optical performance of the gird.
The use of an array of parallel conducting wires to polarize radio waves dates back more than 110 years. Wire grids, generally in the form of an array of thin parallel conductors supported by a transparent substrate, have also been used as polarizers for the infrared portion of the electromagnetic spectrum.
As described in Applicants' prior application, the key factor that determines the performance of a wire grid polarizer is the relationship between the center-to-center spacing, or period, of the parallel grid elements and the wavelength of the incident radiation. If the grid spacing or period is long compared to the wavelength, the grid functions as a diffraction grating, rather than as a polarizer, and diffracts both polarizations (not necessarily with equal efficiency) according to well-known principles. When the grid spacing or period is much shorter than the wavelength, the grid functions as a polarizer that reflects electromagnetic radiation polarized parallel to the grid elements, and transmits radiation of the orthogonal polarization.
The transition region, where the grid period is in the range of roughly one-half of the wavelength to twice the wavelength, is characterized by abrupt changes in the transmission and reflection characteristics of the grid. In particular, an abrupt increase in reflectivity, and corresponding decrease in transmission, for light polarized orthogonal to the grid elements will occur at one or more specific wavelengths at any given angle of incidence. These effects were first reported by Wood in 1902 (Philosophical Magazine, September 1902), and are often referred to as "Wood's Anomalies". Subsequently, Rayleigh analyzed Wood's data and had the insight that the anomalies occur at combinations of wavelength and angle where a higher diffraction order emerges (Philosophical Magazine, vol. 14(79), pp. 60-65, July 1907). Rayleigh developed an equation to predict the location of the anomalies (which are also commonly referred to in the literature as "Rayleigh Resonances").
The effect of the angular dependence is to shift the transmission region to larger wavelengths as the angle increases. This is important when the polarizer is intended for use as a polarizing beam splitter or polarizing turning mirror.
A wire grid polarizer is comprised of a multiplicity of parallel conductive electrodes supported by a substrate. Such a device is characterized by the pitch or period of the conductors; the width of the individual conductors; and the thickness of the conductors. A beam of light produced by a light source is incident on the polarizer at an angle .THETA. from normal, with the plane of incidence orthogonal to the conductive elements. The wire grid polarizer divides this beam into a specularly reflected component, and a non-diffracted, transmitted component. For wavelengths shorter than the longest resonance wavelength, there will also be at least one higher-order diffracted component. Using the normal definitions for S and P polarization, the light with S polarization has the polarization vector orthogonal to the plane of incidence, and thus parallel to the conductive elements. Conversely, light with P polarization has the polarization vector parallel to the plane of incidence and thus orthogonal to the conductive elements.
In general, a wire grid polarizer will reflect light with its electric field vector parallel to the wires of the grid, and transmit light with its electric field vector perpendicular to the wires of the grid, but the plane of incidence may or may not be perpendicular to the wires of the grid as discussed here.
Ideally, the wire grid polarizer will function as a perfect mirror for one polarization of light, such as the S polarized light, and will be perfectly transparent for the other polarization, such as the P polarized light. In practice, however, even the most reflective metals used as mirrors absorb some fraction of the incident light and reflect only 90% to 95%, and plain glass does not transmit 100% of the incident light due to surface reflections.
Applicants' prior application shows transmission and reflection of a wire grid polarizer with two resonances which only affect significantly the polarizer characteristics for P polarization. For incident light polarized in the S direction, the performance of the polarizer approaches the ideal. The reflection efficiency for S polarization is greater than 90% over the visible spectrum from 0.4 .mu.m to 0.7 .mu.m. Over this wavelength band, less than 2.5% of the S polarized light is transmitted, with the balance being absorbed. Except for the small transmitted component, the characteristics of the wire grid polarizer for S polarization are very similar to those of a continuous aluminum mirror.
For P polarization, the transmission and reflection efficiencies of the wire grid are dominated by the resonance effect at wavelengths below about 0.5 .mu.m. At wavelengths longer than 0.5 .mu.m, the wire grid structure acts as a lossy dielectric layer for P polarized light. The losses in this layer and the reflections from the surfaces combine to limit the transmission for P polarized light.
Applicants' prior application also shows the calculated performance of a different type of prior-art wire gird polarizer, as described by Tamada in U.S Pat. No. 5,748,368. As stated above, an index matching fluid has been used between two substrates such that the grid is surrounded by a medium of constant refractive index. This wire grid structure exhibits a single resonance at a wavelength about 0.52 .mu.m. There is a narrow wavelength region, from about 0.58 to 0.62 .mu.m, where the reflectivity for P polarization is very nearly zero. U.S Pat. No. 5,748,368 describes a wire grid polarizer that takes advantage of this effect to implement a narrow bandwidth wire gird polarizer with high extinction ratio. The examples given in the Tamada patent specification used a grid period of 550 nm, and produced a resonance wavelength from 800 to 950 nm depending on the grid thickness, conductor width and shape, and the angle of incidence. The resonance effect that Tamada exploits is different from the resonance whose position is described above. While the two resonances may be coincident, they do not have to be. Tamada exploits this second resonance. Furthermore, there are thin film interference effects which may come into play. The bandwidth of the polarizer, where the reflectivity for the orthogonal-polarized light is less than a few percent, is typically 5% of the center wavelength. While this type of narrow band polarizer may have some applications, many visible-light systems, such as liquid crystal displays, require polarizing optical elements with uniform characteristics over the visible-spectrum wavelengths from 400 nm to 700 nm.
As described in Applicants' prior application, a necessary requirement for a wide band polarizer is that the longest wavelength resonance point must either be suppressed or shifted to a wavelength shorter than the intended spectrum of use. The wavelength of the longest-wavelength resonance point can be reduced in three ways. First, the grid period can be reduced. However, reducing the grid period increases the difficulty of fabricating the grid structure, particularly since the thickness of the grid elements must be maintained to ensure adequate reflectivity of the reflected polarization. Second, the incidence angle can be constrained to near-normal incidence. However, constraining the incidence angle would greatly reduce the utility of the polarizer device, and preclude its use in applications such as projection liquid crystal displays where a wide angular bandwidth centered on 45 degrees is desired. Third, the refractive index of the substrate could be lowered. However, the only cost-effective substrates available for volume manufacture of a polarizer device are several varieties of thin sheet glass, such as Corning type 1737F or Schott type AF45, all of which have a refractive index which varies between 1.5 and 1.53 over the visible spectrum.
Thus, there exists a need for a wire grid polarizer which performs optimally when imbedded or encased. In addition, there exists a need for such a wire grid polarizer, particularly for use in visible light systems requiring broad wavelength bandwidth. In addition, there is a need for such a polarizer structure in which the longest-wavelength resonance point can be eliminated or shifted to a shorter wavelength.